1. Field of the Invention
This invention relates to microstructured filters for fluids.
2. Background of the Invention
Various filters are known, in which the filter medium has micropores down into the submicrometer range, the pore size being statistically distributed in dependence on the material. The external dimensions of filter media of this kind are powers of ten greater than the mean pore diameter and experience has shown that they cannot readily be made as small as may be desired.
Micro-apertured metal strips which are used for screen printing are also known, up to a thickness of 100 μm, comprising for example nickel, provided with holes which are uniformly distributed over the strip, the diameter of the holes being some micrometers. These strips are produced for example galvanically. Metal strips of this kind cannot be assembled with microstructured components.
European Patent Specification No. 0 231 432 describes a cross-flow microfilter to which the fluid to be filtered is fed and out of which a concentrate flow and a filtrate flow are taken. Disposed between the chamber into which the fluid flows and the collecting chamber for the filtrate is a row of webs or lands between which there are passages. The row of webs and passages forms the microfilter. The direction of the passages is inclined through an angle of 90° to 135° with respect to the direction of flow of the fluid/concentrate. The supplied fluid which goes into the concentrate flows past the row of webs. The filtrate is collected in a plurality of chambers and leaves the filter either perpendicularly to the filter surface or in the filter surface in a plurality of passages which extend between the passages for the concentrate.
International Patent Specification No. WO 93/11862 discloses a micromechanical filter which is constructed from three layers. Disposed on the closed base layer in given regions is an intermediate layer and disposed thereon is a cover layer with openings that are elongate in a region-wise manner. The intermediate layer is missing in parallel relationship to one or both longitudinal sides of the openings. In those regions, the cover layer is arranged in a cantilever or overhung configuration. Disposed under the cantilever part of the cover layer, adjoining the opening, is a shallow slot which is as thick as the intermediate layer and as long as the elongate opening. The filtrate flows through that slot into the filtrate collection chamber which is thicker than the intermediate chamber. The cover layer contains a large number of the elongate openings which are arranged in row-wise manner parallel to each other. The rows of slots can be arranged in a meander configuration in the cover layer. The fluid flows through a plurality of openings perpendicularly to the filter surface into a plurality of inlet chambers and is removed from a plurality of filtrate collecting chambers through a plurality of openings perpendicularly to the filter surface. The layers of that filter can be made from silicon, plastic material or metal and are structured by etching, embossing or mechanical processing or machining, while methods involving thin film technology and metal deposition out of the vapor phase can be included.
These, and other, previously proposed devices suffer from a number of problems. For example, it has been noted that at least some of the previously proposed devices are unduly susceptible to blockage whereupon the device can then cease to function. In an effort to alleviate this problem it has been proposed to provide a larger filter, but these larger filters have an undesirably large dead volume. Also, some of the previously proposed devices are unduly complicated, and thus expensive and time consuming to manufacture. In addition, some of the previously proposed devices are such that they cannot easily be assembled with other microstructured components.
Accordingly it is an object of the invention to provide a microstructured filter for a fluid that alleviates one or more of the problems described herein.